Central neuronal mechanisms responsive to pain have been described at spinal and supraspinal levels. The nature of these responses and how they interact with each other remains only partially understood. There is a particular lack of knowledge about the central effects of visceral pain and its influences on somatosensory functions. One of the difficulties in the study of pain mechanisms is inherent in the nature of the commonly used noxious stimuli, the concomitant excitation of afferent activity unrelated to pain. This difficulty may be avoided by using a naturally occurring polypeptide, bradykinin which has well known pain producing properties when administered intra-arterially. A study of pain mechanisms at spinal and supraspinal levels, using bradykinin as the natural, adequate pain stimulus for visceral and somatic structures may lead to an improved understanding of the neurophysiology of pain and may facilitate the study of anesthetic and analgesic actions.